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Physicists in the US have done calculations that suggest "Diraccones" exist in thin films made of bismuth and antinomy. This is anunexpected result because until now such cones have only been seenin graphene and its cousin materials graphynes. Although thepredictions have not been tested in the lab – and only applyat ultra-low temperatures – the researchers are hopeful thatthe films might find use in next-generation electronics devices. Dirac cones are features in the electronic band structure of a 2Dmaterial where the conduction and valence bands meet in a singlepoint at the Fermi level. The bands approach this point in a linearway, which means that the effective kinetic energies of theconduction electrons (and holes) are directly proportional to theirmomenta. This unusual relationship is normally only seen forphotons, which are massless, because the energies of electrons andother particles of matter at non-relativistic velocities usuallydepend on the square of their momenta. The result is that theelectrons in Dirac cones behave as though they are relativisticparticles with no rest mass, travelling through the material atextremely high speeds – a property that could be exploited tomake ultrafast transistors. Better than graphene? Until now, Dirac cones have only been seen in graphene (and morerecently "graphynes"), which has two such (unequal) cones, butShuang Tang and Mildred Dresselhaus at the Massachusetts Instituteof Technology have created a mathematical model that suggestssingle Dirac cones can exist in 2D bismuth–antinomy films."Not only that, but we expect that the single cone found inbismuth–antimony can do all the things that the graphene[Dirac cones] can do, and better!" says Tang. "For example, theDirac cones in graphene are isotropic, so the variety of devicesthat can be made from this material is limited. However, Diraccones with a wide range of anisotropies can be constructed inbismuth–antimony films, something that could increase thetypes of potential devices that might be fabricated." Bismuth–antimony films with Dirac cones conduct electricityextremely well while having low thermal conductivity, twoproperties that make them promising thermoelectric materials– substances that convert heat into useful electrical energy.Tang and Dresselhaus say that they could now make quasi-Dirac oneswith different bandgaps, which would greatly increase the entropycarried per charge carrier (a measure of thermoelectricperformance) in the material without destroying the electricalconductivity. "Basically, for thermoelectrics you need to have atemperature difference across a sample if you want to produce anelectric current," explains Tang. "In this respect,bismuth–antimony films could be especially interesting forapplications in space stations and satellites where electricitycould be generated by exploiting the difference between thespacecraft's Sun-facing and shaded sides." Electronics applications According to Tang, the films could also form the base material fornext-generation electronic devices. "Electron speeds in devicesmade of bismuth–antimony would be hundreds of times greaterthan those in current silicon devices," he says. "At the same time,the fact that different anisotropies of cones can be elaboratedhere means that different devices could be made out of the sameclass of material, which would greatly save on manufacturingcosts." The MIT group's calculations have been published in Nano Letters . I am an expert from sport-sunglass.com, while we provides the quality product, such as China Reading Glasses , Military Glasses, Children Sunglasses,and more.
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